Heretofore, a color image forming apparatus of an electrophotographic type using an intermediate transfer belt has been known. The color image forming apparatus transfers a toner image formed on an image carrier which is a photoconductor, onto an intermediate transfer member (such as an intermediate transfer belt), and transfers this toner image on the intermediate transfer member onto a transfer material (also called a recording paper or sheet). In other words, the apparatus transfers the toner image which is charged to a predetermined polarity and formed on the photoconductor, onto the intermediate transfer belt using electrostatic force, and then transfers this toner image on the intermediate transfer belt onto the transfer material using electrostatic force. With such a color image forming apparatus, the toner charge amount on the intermediate transfer member may vary depending on the number of primary transfers or environmental conditions, so that various image defects are likely to occur in the secondary transfer to the transfer material from the intermediate transfer member holding the toner image with the charge amount widely dispersed.
Such an image forming apparatus using the intermediate transfer belt is broadly used as a color image forming apparatus, because the apparatus can sequentially superimpose individual toner images formed on the photoconductor onto the intermediate transfer belt, and can further transfer the superimposed toner image onto the transfer material in batch.
In this color image forming apparatus, it is difficult to obtain a high quality image due to the secondary transfer failure in the secondary color (or more), even though the secondary transfer performance is good in the primary color. This is because the toner charge amount on the intermediate transfer belt varies depending on the number of primary transfers, environment or other conditions, various image defects are likely to occur in the secondary transfer from the intermediate transfer belt to the transfer material.
To cope with the above disadvantage, there is proposed a technology, for the purpose of correcting the charge amount of the toner image, that charges the toner image which is primarily transferred onto the intermediate transfer belt by corona discharge in an pre-secondary-transfer electricity removal unit of AC, DC and the like, to equalize the toner charge amount (see, for example, Patent Documents 1, 2, and 3).
Further, in order to prevent such failures as density irregularities due to the lack of transfer charge that occurs when the toner adhering amount is large and the toner layer potential is high and a discharge in the transfer charge increased, a technology for applying an electricity removal processing to the toner image on the intermediate transfer belt before the secondary transfer is also proposed.
[Patent Document 1] Japanese Patent Publication Laid-Open No. HEI 10-274892
[Patent Document 2] Japanese Patent Publication Laid-Open No. HEI 11-143255
[Patent Document 3] Japanese Patent Publication Laid-Open No. 2003-57959
In Patent Documents 1 and 2, the charge amount of the toner on the intermediate transfer member is equalized at a larger value, so that when the environmental humidity is low or the secondary transfer is carried out onto a transfer material having a high resistance value such as heavy paper, image defects are likely to occur in the discharge due to the potential increase in the transfer material, and when the transfer voltage is suppressed to prevent such image defects, a portion in which the total charge in the toner layer is large becomes deficient in the transfer electric field, thereby the density irregularities and toner scattering toward around the image will occur.
Thus, in order to prevent the density irregularities due to the lack of the transfer charge occurring with a large amount of adhering toner, the discharge in the charge increased and other related failures, it is conceivable to carry out the electricity removal from the toner image on the intermediate transfer member before the secondary transfer.
However, in a case in which a plurality of levels are provided for the system speed depending on the thickness of the transfer material and other factors, the electricity removal efficient of the toner image on the intermediate transfer belt varies depending on the speed of the intermediate transfer belt, and this causes a problem of insufficient electricity removal, over electricity removal and the like, particularly, relative to the image portion having a large amount of adhering toner.
Further, the electricity removal efficiency of the toner layer charge varies depending on the operational environment surrounding the apparatus, so that the toner charge amount after the removal of electricity also varies, thereby causing a problem that it is difficult to obtain a better toner charge amount for the secondary transfer.